As optical fiber communications channels increasingly replace metal cable and microwave transmission links, integrated optical devices for directly processing optical signals become increasingly important. A particularly useful approach to optical processing is through the use of integrated glass waveguide structures formed on silicon substrates. The basic structure of such devices is described in C. H. Henry et al., "Glass Waveguides on Silicon for Hybrid Optical Packaging", 7 J. Lightwave Technol., pp. 1530-1539 (1989), which is incorporated herein by reference. In essence a silicon substrate is provided with a base layer of SiO.sub.2, and a thin core layer of doped silica glass is deposited on the oxide. The core layer can be configured to a desired waveguide structure--typically 4-10 micrometers wide--using standard photolithographic techniques, and a layer of doped silica glass is deposited on the core to act as a top cladding. Depending on the precise configuration of the waveguide, such devices can perform a wide variety of functions such as beam splitting, tapping, multiplexing, demultiplexing and filtering.
Mach-Zehnder interferometers are key components in integrated glass waveguide devices. In essence, a MZI comprises a pair of couplers connected by two waveguides of different length. FIG. 1, which is prior art, illustrates a conventional MZI comprising two waveguides 11 and 12 disposed on a substrate 13 such as silicon. The two waveguides are closely adjacent at two regions 14 and 15 to form two directional couplers (typically 3 dB couplers) which split and recombine light traveling on the two waveguides. The lower waveguide 11, referred to as the lower arm, has an optical path length L. The upper waveguide 12 has a longer path length L+.DELTA.L and a configuration more curved than the lower arm 11.
In operation, the MZI acts as a simple filter. Transmission maxima occur at wavelengths .lambda..sub.i traveling over arms 11 and 12 that reach the output 3 dB coupler 15 with a phase difference .DELTA..phi.=2.pi.m where m is an integer value. Transmission minima occur at .lambda..sub.j where .DELTA..phi. is an odd multiple of .pi.. This periodic behavior makes the MZI suitable as a simple optical filter, a wavelength division multiplexer and, as will be shown, a wavelength reference. Sequences of suitably configured MZIs can be used to produce filters of many different characteristics. See U.S. Pat. No. 5,596,661 issued to C. H. Henry et al. on Jan. 21, 1997, which is incorporated herein by reference.
A difficulty with conventional MZIs is that they require relatively long, large areas of substrate surface. To obtain a longer optical pathlength, one of the arms is typically curved. However the amount of curvature is limited by bend loss and dispersive effects. When traversing a curve, light is lost and optical modes are shifted radially outward. A mode loosely bound to the waveguide core (TM) will experience greater loss and a greater outward shift than a mode more tightly bound (TE). As a consequence, radii of curvature are kept large and considerable size is required to achieved desired differential path .DELTA.L.
These geometrical constraints are particularly troublesome for applications of the MZI as a wavelength reference source. Because the MZI is large, the laser light source and the MZI typically must be on separate substrates. But the different substrates, in turn, produce different polarization shifts (due mainly to stress birefringence). Thus the large size requires additional devices to compensate polarization differences. Accordingly there is a need for more compact MZI devices.